| HD 189733b |
The turbulent alien world, cataloged HD 189733b, is one of the nearest exoplanets to Earth that can be seen crossing the face of its star. It has been intensively studied by Hubble and other observatories, and its atmosphere is dramatically changeable and exotic. (Science Daily)
If seen directly HD 189733b would look like a "deep blue dot," reminiscent of Earth's color as seen from space. But that's where all comparison ends. The planet's daytime atmosphere is nearly 2,000 degrees Fahrenheit, and it possibly rains glass -- sideways -- in howling 4,500-mile-per-hour winds.
The cobalt blue color doesn't come from the reflection of a tropical ocean, but rather from a hazy blow-torched atmosphere and perhaps from high clouds laced with silicate particles. The condensation temperature of silicates could form very small drops of glass that would scatter blue light more than red light.
| Neptune |
Hot Jupiter class planets are chemically some of the more interesting in our area of space. Neptune, a cold gass planet, is deep blue as well but for very different reasons.
And Carl Sagan's sense that finding another pale blue dot would signal the discovery of an Earth-like planet is not necessarily the case.
It is difficult to know exactly what causes the color of a planet's atmosphere, even for solar system planets. For example, Jupiter is reddish due to unknown color-carrying molecules. Venus does not reflect ultraviolet (UV) light due to an unknown UV absorber in the atmosphere.
Earth looks blue from space because the oceans absorb red and green wavelengths more strongly than blue light. In addition, the oceans reflect Earth's blue sky where the shorter blue wavelengths of sunlight are selectively scattered by atmospheric oxygen and nitrogen molecules in a process called Rayleigh scattering.